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BIOMEDICAL IMPORTANCE

The efficient delivery of oxygen from the lungs to the peripheral tissues and the maintenance of tissue reserves to protect against anoxic episodes are essential to health. In mammals, these functions are performed by the homologous heme proteins hemoglobin and myoglobin, respectively. Myoglobin, a monomeric protein of red muscle, binds oxygen tightly as a reserve against oxygen deprivation. The multiple subunits of hemoglobin, a tetrameric protein of erythrocytes, interact in a cooperative fashion that enables this transporter to offload a high proportion of bound O2 in peripheral tissues while simultaneously retaining the capacity to bind it efficiently in the lungs. In addition to delivering O2, hemoglobin scavenges the waste products of respiration, CO2 and protons, for transport to and ultimate disposal in the lungs. Oxygen delivery is enhanced by the binding of 2,3-bisphosphoglycerate (BPG), which stabilizes the quaternary structure of deoxyhemoglobin. Hemoglobin and myoglobin illustrate both protein structure–function relationships and the molecular basis of genetic disorders such as sickle cell disease and the thalassemias. Cyanide and carbon monoxide kill because they disrupt the physiologic function of the heme proteins cytochrome oxidase and hemoglobin, respectively.